The present invention relates to an ion beam buncher - debuncher having asymmetrical gaps and operating in a wide velocity range. It is used in ion acceleration installations.
It is known that an ion beam buncher or debuncher is constituted by a resonant structure supplied by a high frequency or hyperfrequency source and traversed by an ion beam in such a way that the electrical field established in the structure modulates the velocity of the ions in an appropriate manner.
In a buncher, the velocity modulation has the effect of accelerating slow ions more than fast ions, which permits a rebunching in a bunch with a limited spatial extension at a given distance from the buncher. The velocities of the different ions constituting a bunch are then distributed in a wider range. Bunchers are used in ion acceleration systems when it is for example desired to carry out transit time experiments or any injection into a high frequency accelerator.
In a debuncher, the modulation has the effect of increasing the low velocities and decreasing the high velocities, making it possible to reduce the velocity dispersion of the ions. Such an apparatus is used when it is desired to employ monoergic ions and when no particular significance is attached to the width of the ion bunches.
FIGS. 1 and 2 give a general idea of the construction and operating principles of said two apparatuses.
In part (a) of FIG. 1 is shown a resonant structure constituted by a wall 2 closed at its two ends by lateral faces 4 and 6, respectively traversed by a supply pipe 8 and a discharge pipe 10 for an ion beam 12. The structure also comprises a sliding tube 14 connected to wall 2 by a conductive support 16. The sliding tube is separated from pipes 8 and 10 by two identical gaps I.sub.1 and I.sub.2, all the said members being conductive and for example of metal.
Part (b) of FIG. 1 illustrates the electrical diagram of the structure shown in part (a). The two pipes 8 and 10 are connected to earth (or more generally to a reference potential) and the sliding tube 14 is brought to an alternating current voltage V, due to the high frequency or hyperfrequency field in the structure (said voltage V being countered from the reference potential). Each of the gaps I.sub.1 and I.sub.2 has a length 1 and their centres are spaced by length L. A same average electrical field V/1 is therefore present in both these gaps.
The diagrams of FIG. 2 illustrate the operation of the device of FIG. 1. The average velocity ions enter the gap I.sub.1 at time t.sub.o (part a), the distance z which they cover being plotted on the ordinate as a function of the times appearing on the abscissa. During their transit of gap I.sub.1 these ions are subject (b) to an electrical field E.sub.o, said field slightly modifying their velocity (this modification of velocity is generally small compared with the true velocity). Ions which are faster than those indicated hereinbefore reach gap I.sub.1 at time t.sub.1 prior to t.sub.o. They are subject to the action of a field which is weaker than E.sub.o. Conversely, the slower ions only reach gap I.sub.1 at time t.sub.2 &gt;t.sub.o and the latter are subject to a stronger field E.sub.2 than E.sub.o. The relative magnitudes of the fields are therefore such that the slower ions are able to catch up the faster ions, this constituting buncher operation.
This mechanism naturally assumes that the electrical field increases in time in an appropriate manner. In practice, it is rate for the ideal linear modulation to be used, rather there is a sinusoidal modulation or a sum of sinusoidal modulations which are much easier to obtain, said modulation only being used for a substantially linear portion. The ions which penetrate the gap I.sub.1 at periods where the field does not have the appropriate variations are clearly not rebunched. However, in the case of the others, there is a rebunching at a distance z.sub.o from gap I.sub.1.
In an ion debuncher, the mechanism is the same, except that it tends to reduce the energy deficit of the slow ions and reduce the energy excess of the fast ions. A debuncher placed at a distance z receives the ions at time t'.sub.o and applies a field E'.sub.o to them. The faster ions have reached the interaction gap of the debuncher at time t'.sub.1 prior to t'.sub.o. They are subject to a field E'.sub.1 which is weaker than E'.sub.o. As for the slower ions, which reach the debuncher at t'.sub.1, they are subject to a field E'.sub.2 which is stronger than E'.sub.o in the interaction gap. On leaving the debuncher, the ions have a substantially identical velocity, but correlatively they occupy an extensive portion in space.
In both a buncher and a debuncher, the gaps where the ions are subject to the action of the electrical field must be sufficiently short for the transit time of the ions to be less than the half-cycle of the field. If v is the velocity of the ions and T the cycle, it is necessary to have 1/v&lt;T/2.
To explain this, it is pointed out that the voltages which are normally encountered in ion beam bunchers are defined by two requirements: the velocity modulation supplied to the beam must be low with respect to the velocity of the said beam and the accelerating voltage must be high with respect to the natural fluctuations of the beam. In practice, voltages of the order of a few dozen kilovolts or lower are used.
The voltages used in debunchers are of the same order of magnitude as the energy dispersion of the beam and can be between approximately 10 and approximately 100 kV.
In connection with said apparatuses, reference can be made to the article by E. L. HUBBARD et al entitled "Heavy ion linear accelerator", published in the Journal "The review of scientific instruments", Vol. 32, no. 6, June 1961, p.621 and the article by J. S. SOKOLOWSKI et al entitled "Status report on Stanford's superconducting heavy ion linear project", published in the Journal "IEEE Transactions on nuclear science", Vol. NS-24, No. 3, June 1977, p. 1141 and finally the article by B. CORK entitled "Proton linear accelerator injector for the Bevatron", and published in the Journal "The review of scientific instruments", Vol. 26, No. 2, February 1955, p. 210.
After describing these general points, it is possible to define the invention relative to the prior art. The structure shown in FIG. 1 is the closest prior art structure to that of the invention. It can be considered that it is formed by a first part constituted by tube 16, faces 4 and 6 and wall 2, said part being equivalent to a .lambda./4 resonant line, if .lambda. is the wavelength of the electromagnet field introduced into the structure and the second part constituted by gaps I.sub.1 and I.sub.2, which are zones having a capacitive nature.
The interest of such a structure is that it has small overall dimensions (less than .lambda./4), whilst the structures with a single interaction gap the dimensions are of the order of .lambda./2, which becomes prohibitive for working frequencies below 100 MHz (the half-wave length is then equal to 1.5 m).
This prior art structure makes it necessary for the actions exerted by the electrical field on the ions in the two interaction gaps to be of a cumulative nature. This implies that the ions transit the distance L separating these two gaps in a time which is an uneven multiple of the half-cycle T of the field. Thus, this prior art structure only functions correctly if the ions have a velocity close to 2L/T (or a multiply of this velocity.
This constraint made on the velocity of the ions is prejudicial in most applications of bunchers and debunchers. Thus, said apparatuses are generally used in installations comprising, for example and successively, an ion source, an injector, a first accelerator (for example of the Van der Graaf type) and a second accelerator (for example of the linear type). However, in such installations, it is often necessary to vary the energy of the ions, which involves modifying their velocity or changing from one type of ions to another with the energy constant, which also leads to a modification in their velocity.
It is not therefore possible to use the apparatuses described hereinbefore in all cases and their dimensions must be modified as a function of need, which is far from convenient.
Admittedly, devices are known having a single acceleration gap and which do not have the above disadvantage, due to the fact that there is only one gap. However, as has been stressed hereinbefore, these devices have the major disadvantage of large overall dimensions, which increase as the frequency decreases.